Venting systems of the type described above are used in passenger cars in order to reduce the emission of fuel in gaseous form to the environment. The canister thus functions to absorb fuel fumes which have to be evacuated from the tank when the latter is filled up with gasoline and the free space above the fuel level decreases. Normally, the canister is, in turn, controllably connected with the engine suction or intake system, for emptying the canister of absorbed fuel during engine operation. Such a system is described in the British Patent Application No 2,286,182.
In the system according to the above British Patent Application, the connection path, normally a line in the form of a conventional pipe between the tank and the canister, must be sized so as to be able to evacuate those gas volumes from the tank that are replaced with fuel at the same rate as they are filled with the fuel, in order not to build up a counter-pressure that will shut off the orifice of the gasoline pump prematurely. When filling is then finished, and the car is operated, the fuel is consumed, causing the free volume above the fuel to increase, but the rate of change of the free volume is considerably smaller than during filling up the tank, and the flow of air to the tank needed to avoid a vacuum therein is relatively small. For this flow, the line is thus clearly over-sized. When emptying the canister, a line is normally used (see line No. 15 in British Patent Application No. 2,286,182) between the canister and the open air for replacing the gases being sucked from the canister by the engine, such that substantially no gases are taken from the tank and, in general, no flow is created from or any vacuum in the tank.
When operating vehicles having systems of this kind, fuel fumes are forced into the line and the canister due to e.g. decreasing ambient atmospheric pressure or decreasing exterior temperature, especially when travelling a distance including substantial level changes. As the line between the tank and the canister is sized to allow large flows, a rapid displacement of relatively large quantities of air, in some cases saturated with fuel, may occur, causing fuel to condense in the line and/or in the canister instead of being absorbed by the canister contents. This will cause operational disturbances, in the worst case serious enough to stall the engine due to an incorrect fuel/air mixture (too rich).
An object of the present invention is to provide an improved device for fuel tanks, eliminating the problems described above.